Vol. 60, Issue 5, 1112-1120, November 2001

Mpl Ligand Increases P2Y1 Receptor Gene Expression in Megakaryocytes with No Concomitant Change in Platelet Response to ADP

Béatrice Hechler, Paul Toselli, Catherine Ravanat, Christian Gachet, and Katya Ravid

Department of Biochemistry, Whitaker Cardiovascular Institute, Boston University School of Medicine, Boston, Massachusetts (B.H., P.T., K.R.); Institut National de la Santé et de la Recherche Médicale U311, Etablissement Français du Sang-Alsace, Strasbourg, France (C.R., C.G.)

The P2Y₁ receptor is responsible for the initiation of platelet aggregation in response to ADP and plays a key role in thrombosis. Although this receptor is expressed early in the platelet lineage, the regulation of its expression during megakaryocyte differentiation is unknown. In the mouse megakaryocytic cell line Y10/L8057, we detected P2Y₁ mRNA of three sizes (2.5, 4.4, and 7.4 kb). These cells have previously been shown to respond to Mpl ligand, the pivotal regulator of megakaryocytopoiesis, by increasing their expression of differentiation markers. Mpl ligand enhanced levels of P2Y₁ mRNAs in Y10/L8057 cells and this effect was selective: the same cytokine did not increase levels of A2a adenosine receptor mRNA. Although Mpl ligand did not affect the short half-lives of the P2Y₁ mRNAs, it enhanced transcription of the P2Y₁ gene. It also increased cell size and the number of cell surface P2Y₁ receptors, but not P2Y₁ receptor density. Injection of Mpl ligand into mice up-regulated P2Y₁ receptor mRNAs in megakaryocytes, as shown by in situ hybridization. However, platelets isolated from these mice did not exhibit a higher P2Y₁ receptor density or increased reactivity to ADP. This correlates with the finding that Mpl ligand increases GPIIb mRNA in megakaryocytes but not the density of the protein per platelet. Thus, the enhancement of P2Y₁ receptor expression induced by Mpl ligand in megakaryocytes may be an integral feature of their differentiation, whereas clinical use of this compound might not be associated with platelet hyper-reactivity to ADP.